ENGINEERING TRIPOS PART IIA, 2012-13
MODULE 3C1: Materials Processing and Design
MANUFACTURING ENGINEERING TRIPOS PART IIA, 2012-13
PAPER 3P1: Materials into products
HANDOUT 2: CASTING
1. Processes, selection and design
1.1 Casting processes
1.2 Process selection
1.3 Design of castings
2. Solidification theory
2.1 Revision of nucleation theory
2.2 Solidification mechanisms
2.3 Solidification of alloys
3. Microstructure of castings
3.1 Grain structure
3.2 Chemical inhomogeneity
3.3 Porosity
3.4 Casting alloys
Essential Revision: Phase diagrams, phase transformations, shaping processes
IB Materials notes + Teach Yourself Phase Diagrams (www-materials.eng.cam.ac.uk/typd)
Ashby, Shercliff & Cebon: Materials: engineering, science, processing and design
(Ch. 18, 19)
Ashby & Jones: Engineering Materials II
Other References for Casting:
Edwards L and Endean M. Manufacturing with Materials (CUED JA146)
Waters, TF. Manufacturing for Engineers (CUED BN204)
Campbell, J. Castings (Lots of technical detail) (CUED JO41)
H.R Shercliff
(C.Y. Barlow)
October 2012
1
1. PROCESSES, SELECTION AND DESIGN
1.1 Casting processes
Typical casting process and terminology: Sand Casting
1. A solid re-usable pattern (often wooden) is
made of the component.
2. Sand with a small amount of resin binder is
packed around the pattern in a box called a drag.
3. The drag is inverted and the pattern is lifted
out, leaving a cavity. In-gates and runners may
be carved or moulded into the sand.
4. Interior detail may be produced by inserting a
core (also moulded out of sand) into the cavity.
The upper part of the mould (the cope) is
formed from sand, incorporating a pouring
basin, a sprue, vents, risers/feeder heads (either
moulded from patterns – e.g. runner pin and
riser pin shown – or which may be carved in.
5. Mould bolted together, metal poured in.
Once the casting has solidified, the mould
is removed and the sand mould and any
cores broken up and brushed out.
The casting is fettled: cutting off the
runner, ingate, sprue, risers and feeder
head. The parting line of the mould
may also leave a ridge which must be
ground off.
2
Overview of Casting Processes
Melt
Transfer into mould
Refractory (ceramic) crucible
“Clean” heat sources:
Electric furnace, or RF
induction
(can be under vacuum or in an
inert gas atmosphere)
or
Oil/gas-fuelled furnace
Pour under gravity
or
Force under high pressure
into mould
or
Use inert gas pressure
(controlled atmosphere) to
force metal into mould
Check composition
before casting; additives
to refine grain size or
modify structures
Remove from
mould
Permanent mould
(e.g. ingot casting,
continuous casting,
die casting):
open mould, remove
part, clean mould and
re-use
Permanent pattern
(e.g. sand casting):
one-off moulds,
destroy on removal
Solidify: a few
seconds for small
parts; days for large
Classification of casting processes
Ingot or Continuous
Casting
Continuous casting
for most high-volume
steel; “direct chill”
(DC) casting for
wrought aluminium
alloys.
Ingot casting
(permanent mould)
used for lower
volume alloys.
Post-processing:
Homogenisation
+ Thermomechanical
processing, e.g.
hot/cold rolling,
forging, extrusion
+ heat treatment
Shaped Casting
(i.e. solidify to near net-shape)
Permanent mould casting
Permanent pattern casting
Simple shapes: no re-entrant
More intricate shapes: mould
surfaces (need to be able to
for each individual casting is
remove parts from mould).
created around a pattern and
Moulds expensive (tool steel; mould is destroyed as the
may make 103 – 106 castings), casting is removed.
production rates high.
Low setup costs and
production rates.
Typically used for large
numbers of small parts.
Used for larger parts, or
when small production runs.
Examples:
Examples:
Sand casting
Gravity die casting
Investment casting
Pressure die casting
Evaporative mould casting
Centrifugal casting
Post-processing:
“Fettling” (trim solidified feeder channels)
Machine/grind critical areas (improve tolerances and surface
finish around joints, seals, contact surfaces)
Machine/drill features, holes etc.
Some castings heat-treated to improve properties.
3
Examples of permanent pattern (expendable mould) casting
Sand Casting (details above)
Advantages: Versatile, low material and equipment costs, OK for large simple parts;
internal detail possible.
Disadvantages: Poor dimensional accuracy and surface finish; not good for thin sections;
relatively high labour costs; “dirty” process; can’t be used for refractory metals because
sand undergoes phase change.
Investment casting
Something of a hybrid process: by “permanent pattern” we mean that a permanent mould
is made to make an expendable pattern. This pattern is covered in a disposable ceramic/
refractory shell in which the casting itself is produced.
Advantages: Excellent accuracy and surface finish
Disadvantages: Limited to small parts; much more expensive.
4
Full / evaporative mould casting
Closely related variant, using a polystyrene foam pattern.
Advantages: High accuracy and surface finish (especially with small-bead polystyrene);
lighter patterns than wax, so suitable for large parts.
Disadvantages: Labour still quite high
Examples of permanent mould casting
Pressure die casting
Externally applied pressure permits use of higher viscosity fluid, thinner sections, and
minimises waste from runners, risers etc). Susceptible to entrapped bubbles due to
turbulence, which can be detrimental to properties – see later.
A common, important process –
often just called ‘die casting’.
Limited to low-melting point
alloys (because the dies must
not distort or wear whilst
making many thousands of
castings).
Common example: zinc die-casting alloy (a low-melting point alloy, Zn + 4Al, 1Cu,
0.05Mg), chosen for ease of processing and cheapness, rather than for good mechanical
properties, e.g. toy models.
(Note equivalent polymer process: injection moulding, the commonest polymer process. It
uses a rotating screw to plasticise the polymer, but the whole screw is translated along the
feeder tube to force the polymer rapidly into the mould.)
Gravity die casting
Variant process using gravity feed (as in sand casting) but with permanent mould in
separable parts, as in pressure die casting.
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Centrifugal casting
Used for axisymmetric hollow parts (e.g. pipes)
(A related process for polymers is Rotational Moulding)
1.2
Process selection
The choice of process depends on a range of factors including:
material
size, shape complexity, section thickness
These are covered by CES (below)
dimensional accuracy, surface finish
number of parts required
mechanical (and other) properties
Properties sensitive to the combination of
material, process and design parameters
Technical attributes
6
Quality attributes
Economic attribute
Process attribute charts from CES, for
metal shaping processes.
The same factors influence the choice
of process class (cast vs. deformation
vs. powder) and the choice of process
variant within a class (e.g. sand cast
Observations (both comparing process classes, and on variants within casting):
- considerable overlap between process classes on mass and section thickness;
- wide variation in precision and surface finish;
- competition between process classes largely driven by economics – but remember
that it may be cheaper to use an inexpensive process (e.g. sand casting) followed by
a local machining), rather than a more expensive process.
Casting Alloys versus Wrought Alloys
Casting and thermomechanical (wrought) processes use different, dedicated alloys, as
casting alloys must satisfy separate requirements relating to fluidity, lower melting point,
and solidification microstructure. For example:
- carbon steels (Fe + 0.1-0.8wt% C): hot/cold formed;
cast iron (Fe + 4wt% C): only cast.
- wrought Al alloys: Al + Mg + (Cu, Zn or Si) ( 1-5wt%): hot/cold rolled, extruded;
cast Al alloys: Al +Si (typically 12%) (+ Cu, Mg): only cast.
Castings have historically had poorer mechanical properties than their forged counterparts,
largely because of a tendency to contain porosity, and a high second phase content. Casting
(plus heat treatment) can be the route to high-quality property-critical components (e.g.
internal Al alloy frame of Airbus doors, jet engine nickel alloy turbine blades).
Remember that the properties achieved in a casting are dependent on the coupling between
material, process and design parameters.
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1.3 Design of castings
Solidification rate
Solidification time is important in casting because it affects:
- production rate, and hence process economics
- the resulting microstructure, and hence properties
Chvorinov’s rule states that:
solidification time of a section is proportional to [Volume/Surface area]2
Physical basis:
One application of this is in the design of the feeder heads for (e.g.) sand castings. Most
metals shrink on cooling and solidifying, so moulds are designed to hold a reserve of molten
metal to allow metal to be fed in during solidification. Hence the metal in the feeder head
must solidify last, otherwise parts of the casting may be starved, leading to porosity:
Solidification rate also affects the scale of the microstructure, which may have implications
for mechanical properties. What matters physically is not the total time for solidification, but
the local velocity of the solidification interface. The length-scale of the microstructure (e.g.
the spacing of the plates in eutectics) is inversely proportional to this velocity.
Casting Defects
Defects include cavities, and internal or surface cracks. Non-destructive testing (NDT) of
component integrity is routine for larger parts, by such methods as:
Surface: visual examination, die penetrant, magnetic particles
Internal: X-rays, sonic (e.g. C-scan, damping properties)
The formation of defects in casting depends on many factors in the design of the gating
system and the mould, the shape of casting, and the alloy used. The main physical origins
of defects are: porosity and turbulence, fluidity, and shrinkage.
(i) Porosity and Turbulence
Porosity in casting can be on a macroscopic scale (cavities on millimeter scale) or on the
scale of the microstructure (micron scale). The source of porosity is either dissolved gas
released during solidification, or bubbles entrapped by turbulence. Gases may be absorbed
from the atmosphere, or may be injected as part of process (e.g. oxygen is injected in steelmaking to burn off the excess carbon in pig iron, to the lower level needed in the steel,
leaving residual oxygen, CO and CO2 dissolved in the steel). More details in section 3.3.
8
Turbulence may arise close to where metal is poured into mould, or within the mould if there
are changes in section.
(a) Air may be entrapped, leading to the formation of large-scale porosity (blowholes), or
smaller pores.
Pressure die-casting always results in turbulence and porosity because the metal enters the
mould so quickly: the mechanical properties of such castings are notably poor as a result.
Vacuum die-casting, though expensive, improves casting quality – the metal is degassed by
putting it under vacuum before casting.
(b) In sand casting, the mould may be damaged by rapid metal flow, releasing sand into the
casting and causing loss of dimensional accuracy of the casting. Types of mould damage:
(c) The surface of the liquid metal is often oxide covered: with turbulent flow the oxide can
become entrapped in the casting.
Solutions:
Well-designed gating system.
Avoid sudden changes in section thickness.
Vacuum degassing before casting.
(ii) Fluidity: Misruns and Cold Shuts
Solutions:
Redesign running and gating systems (position, size and number of ingates and vents).
Increase fluidity by raising pouring temperature or preheating mould.
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(iii) Shrinkage
Moulds must be designed to allow the semi-solid casting to shrink safely (particularly when
there is a large semi-solid temperature range): semi-solid has very little strength.
When section changes are unavoidable, the
solidification pattern may be altered by the use
of chills, to cause early solidification in
vulnerable regions (e.g. metal insert in sand
mould).
2 SOLIDIFICATION THEORY
2.1 Revision of nucleation theory
A system is characterised by the Gibbs free
energy G = H – TS (H is enthalpy; T absolute
temperature; S is entropy). For any phase, G
varies with T as shown.
G
Liquid
Phase transformations occur when the lines
for two states cross: a system can reduce its
free energy G by shifting to the lower line.
The energy difference between the lines, the
volume free energy, is the driving force.
Solid
Temperature, T
Homogeneous Nucleation
Crystals form spontaneously (by statistical fluctuations) within a liquid which is below its
melting temperature. The system reduces its internal energy U by producing solid:
volumetric (chemical) free energy Gv (which will be a negative number) is the driving
force for solidification. Surface energy SL per unit area must be supplied by the system.
10
For a spherical nucleus:
Net energy change = Surface energy + volume energy
U  4  r 2  SL  4  r 3 Gv
3
Volume free energy Gv 
H T
Tm
, where undercooling T = Tm – T
(Tm = melting temperature).
For a nucleus to be stable its energy must fall as r increases: differentiate U to find
maximum:
2  SL
U
 0 , from which the critical radius r * 
Gv
r
For a typical nucleus of radius r* = 1nm, undercooling = 100K (very large).
Heterogeneous nucleation
Instead of the nuclei atoms forming a spherical cluster, they are attached to a
nucleating surface, e.g. mould wall, impurity (details later).
liquid, L
contact or
‘wetting’ angle 
interface energy SL
solid, S
nucleating surface, N
Note that there are actually three surface energies interacting: SL (as before), and also
those between the nucleating surface and both liquid and solid, NL and NS. The solid
forms a circular area on the nucleating surface, but has displaced the same area that was
previously there between the liquid and the nucleating surface. There is therefore a net
energy change given by the difference in the surface energies (NS – NL), multiplied by the
circular area of contact. The interaction between the competing surface energies is
conveniently captured by the contact or wetting angle, .

Volume of spherical cap = 1  r 3 ( 2  3 cos  cos 3 )
r* 
2  SL
Gv
3
(as before)
Undercooling to achieve critical radius is much smaller – virtually all nucleation in casting
is heterogeneous.
11
Temperature dependence of nucleation rate:
Nucleation
rate
T = Tm – T
Influence of contact angle :
Graph shows ratio of net energy changes at critical radius, U*, for heterogeneous to
homogeneous nucleation.
Good wetting (i.e. low ) is needed for easy
heterogeneous nucleation. Very specific
particles are used in industrial solidification
processes to stimulate heterogeneous
nucleation. These are known as inoculants
(more detail later).
U*HET
U*HOM

2.2 Solidification mechanisms
Stable growth
If molten metal is poured into a mould to cast an ingot, solidification will start as a result of
heterogeneous nucleation on the mould walls. Latent heat is released at the solid-liquid
interface as solid forms. Heat is removed from the melt by conduction through the mould
wall, and the solidification front moves in towards the centre of the ingot.
Consequences:
Temperature of interface is TM
Solidification rate proportional to rate of removal of latent heat
Growth rate determined by rate at which heat is lost from system
Metals solidify easily because solid-liquid interfaces are rough on an atomic scale (even
though they may be smooth on a microscopic scale). This means that there are many sites
available for atoms to attach themselves firmly to the new solid.
Leads to rounded, ‘blobby’ solid particles.
12
Non-metals (e.g. graphite, silicon) generally have an atomically smooth solid-liquid
interface. Growth takes place by steps growing across the solid in an orderly way, and is
much slower than for metals.
Leads to more angular particles: needles; ‘crystals’, snowflakes
The same applies when these non-metals form as a second phase in a casting alloy (e.g.
graphite in cast iron, silicon in Al alloys). This is not good news for toughness – flakes of
brittle phases behave like cracks. The growth can be modified by poisoning: atoms of an
alloy addition which halt the growth of steps, leading to smaller more rounded
“morphology” (size and shape) of the brittle second phase (examples later).
Unstable growth: dendrites
Consider first the solidification of a pure liquid (alloys are considered in the next section).
Nucleation requires supercooling of the liquid, with the first growth of nuclei into liquid
below the equilibrium solidification temperature, Tm. The nuclei release latent heat which
warms up the solid, and the liquid immediately around them, to the melting point.
Overall cooling history:
Away from the nucleus, there is a negative
temperature gradient into the supercooled liquid:
T
time
13
As a result, there is a greater driving force for solidification away from the solid-liquid
interface than at the interface itself. Growth is therefore unstable – if part of the nucleus
extends into the surrounding liquid, it can continue to grow rapidly in that direction. This
occurs spontaneously in particular crystallographic directions, leading to long thin crystals.
The same behaviour may then be repeated with secondary side arms forming on the side of
these crystals.
The distinctive crystal shape formed during solidification is known as a dendrite.
In pure liquids, these are known as thermal dendrites (as they are the result of a thermal
gradient leading to supercooling).
Dendrite growth stops when the residual liquid has all been warmed to the melting point –
further solidification is then by stable growth (thickening of the dendrite arms), governed
by the remote loss of heat to the surroundings.
Once the arms of a given dendrite meet, they form a single grain (since all arms of the
dendrite have a common crystallographic orientation), and their characteristic structure is
not visible – the final structure only reveals the grain boundaries where the crystal
orientation changes. Each nucleus has a random orientation of the crystallographic planes,
and becomes a single grain.
14
2.3 Solidification of Alloys
Concentration gradients in solidification
Consider the solidification of liquid with initial concentration Co.
Idealised phase diagram:
Definitions:
k = partition coefficient (k < 1)
CS /CL = k
where CS and CL are
equilibrium concentrations
Solidification of a long bath of liquid with diffusion in the liquid:
(a) Formation of initial transient
Solidification starts at temperature T0.
For typical solidification timescales, there is essentially no diffusion in the solid, hence the
non-uniform concentration gradient is “locked in”: this is known as segregation.
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(b) Steady state
Solidification temperature is now T1
(c) Final solidification
of liquid
…………
Long bar
Solidification temperature
now undefined, but can be
well below T1
Now consider solidification from both sides of a mould towards the centre (two of these
curves back-to-back). The excess solute from both sides concentrates into the last part to
solidify. This macrosegregation (i.e. on the scale of the whole casting) may lead to poor
mechanical properties (high concentrations of brittle second phases in the grain
boundaries). And if the segregating solute is a gas, this may lead to the formation of
macroporosity in the centre of the casting. More details later.
Constitutional supercooling of alloys
Recall from Part IB Teach Yourself Phase Diagrams:
“Constitution” = phases present, their proportions and compositions
Consider again the steady-state distribution of composition, away from the initial transient
region (figure below). From the phase diagram, the higher the concentration, the lower the
solidus temperature. Hence there is a gradient in the local solidification temperature with
position, which essentially has the same form of the concentration curve, but inverted.
Now we superimpose the actual temperature gradient in the casting (decreasing from right
to left, as heat is conducted out through the mould in this direction). The temperature is
equal to the local solidification temperature at the interface, but there are then two
possibilities, depending on the gradient in the temperature (dT/dx) compared to the
gradient in the local solidification temperature (dTm/dx).
(Make sure that you distinguish the gradient, with x, in Tm from the gradient in T).
16
(i) (dT/dx) > (dTm/dx): T in liquid everywhere above its (local) melting point;
solidification is stable at the interface (leading to a planar solidification front and columnar
grain growth).
(ii) (dT/dx) < (dTm/dx): there is a region ahead of the interface for which T in liquid is
below its (local) melting point; solidification is therefore unstable – regions extending into
this liquid can rapidly solidify ahead, i.e. dendrite formation.
This is known as constitutional supercooling, since in this case it is a composition
gradient coupled with a thermal gradient that leads to supercooling.
This condition can remain throughout steady-state solidification, but only in the small
region just ahead of the growing solid – the “bow wave” of elevated concentration
translates ahead of the interface, at the same speed. A constitutionally supercooled region
is commonly found in the solidification of alloys, so dendritic growth is the usual
solidification mode.
The imposed cooling gradient, dT/dx, depends on many factors:
- the casting alloy (i.e. its thermal properties, and its phase diagram)
- type of mould (i.e. its thermal properties and its size), pouring temperature
- size and shape of the casting
This is a good example of how casting properties depend in a complex coupled way on the
combination of material, process and design parameters.
17
3
MICROSTRUCTURE OF CASTINGS
3.1 Grain Structure
In general, heterogeneous nucleation occurs first on the cool mould walls, with a dense
array of small grains forming the chill zone. As cooling continues, favourably-oriented
nuclei grow inwards to form a columnar zone. The central part may be occupied by an
equiaxed zone, heterogeneously nucleated within the melt. The proportions of the three
structures depend on the alloy and the cooling conditions:
Horizontal
Section
Vertical
Section
No nucleation within
melt
Nucleation both on
walls and within melt
Columnar zone growth mechanism:
Nuclei in the chill zone are
randomly oriented. Competitive
growth takes place – those with
their fast growth direction normal to
the mould wall survive. These form
the columnar zone.
18
No nucleation on walls
It is often desirable to ensure that equiaxed grains form the major part of the casting:
 Finer grain size.
 Reducing the extent of macrosegregation (see below).
 Interfaces between columnar grains also contain high proportions of impurities due to
lateral segregation. Feeding liquid metal into these regions is difficult, so fully
columnar castings can contain interconnected porosity.
Nuclei for grains in the equiaxed zone can arise from various sources:
(a) Oxide or solid metal nuclei formed on surface of melt during pouring
(b) Inoculants (grain refiners): small amounts of specific solid (fine powder) added just
before pouring. Inoculants must have a very low wetting angle to allow easy nucleation.
e.g. addition of 0.05 – 0.1%TiB2 is standard for cast Al alloys.
(c) Turbulence in melt breaks off dendrites, which form stable nuclei in body of melt.
Increase turbulence by vibrating mould – though this is not practical for many processes.
Special process: casting single-crystal jet
engine turbine blades
Grain boundaries accelerate creep (e.g.
providing short circuit diffusion paths). So
components designed to resist stress under
high-temperature conditions are sometimes
made as single crystals. To achieve this, we
need to make one 'seed' crystal grow. Rather
than placing a seed crystal into the casting
mould, we can force the system to perform
the selection of a single seed itself.
Rolls Royce Trent 800 turbine blades are
made from creep-resistant nickel-based
superalloy MAR-M200. These “nimonic”
alloys are fcc, with fast growth directions at
90º to each other. The blades are solidified
by slowly withdrawing downwards from a
furnace, so solidification starts in the helical
grain selector, or ‘pigtail’. This guarantees
that only one crystal orientation survives as
the casting solidifies from the bottom. Tight
control of the thermal conditions around the casting is required to ensure that the blade
solidifies from this end only, without further nucleation from the mould.
Interior channels are also created by using a "core", held in place before and during casting
by positioning pins (P’Pins). The core is dissolved out after casting.
19
3.2 Chemical inhomogeneity
Cast structures in alloys all have some degree of partitioning of elements (i.e. segregation).
(a) Macro-scale: If the columnar zone extends through the whole casting, there will be
inhomogeneity on the scale of the casting. Impurities accumulate on the centre-line of the
casting, leading to a plane of weakness.
Solution: rather than removing the impurities (expensive) the solution is further alloying, to
trap the impurities in a harmless form throughout the casting.
e.g. all C steels contain sulphur as an impurity; the addition of Mn leads to the formation
of a dispersion of MnS particles thoughout the casting, rather than letting the S segregate to
the grain boundaries, forming brittle FeS.
(b) Medium scale: Equiaxed grains will have some
edge-to-centre segregation (typical grain sizes
100µm up to several mm). Dendritic or columnar
grains will show compositional variation across their
width: the interior of the grains being purer than the
grain boundaries.
Composition gradients can sometimes be revealed by
etching (see figure).
(c) Micro-scale: Segregation occurs between the dendrites arms (primary and secondary).
Primary dendrite arms
The ‘dendrite arm spacing’ refers to the
length scale of the secondary dendrite arms
(typically a few µm), and is often cited as a
key length-scale of the cast microstructure
(i.e. the scale of inhomogeneity that can be
influenced by heat treatment – see below).
Secondary dendrite arms
Segregation leads to different parts of the casting having different mechanical and physical
properties (e.g. yield strength, melting point). The significance of segregation depends on
the length size, and what happens subsequently to the casting:
Large-scale: central impurities may end up as a weak mid-plane when an ingot is rolled.
Medium-scale: grain boundaries may corrode more rapidly, or form coarse precipitates at
grain boundaries, reducing toughness or ductility.
Micro-scale: non-uniform distribution of precipitates in subsequent age-hardening heat
treatments.
20
Homogenisation heat treatment
Homogenisation is an immediate, prolonged “soak” of a casting at high temperature in the
single-phase field (e.g. 24 hours at 530-580oC for Al alloys, with melting point c.630oC).
This requires bulk diffusion of the components that are distributed inhomogeneously. In
most cases the alloying additions form substitutional solid solutions (rather than interstitial),
and are therefore inherently slow to diffuse – hence high temperatures and long hold times
are needed.
The degree of homogenisation can be estimated using the rule-of-thumb diffusion equation:
x2 = Dt
where diffusion of species with diffusion rate D takes place over a characteristic distance x in
time t, and D = Do exp (-Q/RT) (cf. Materials Databook).
The practical limits to temperature and time set a limit on the length-scale that can
reasonably be homogenised (see Examples Paper 1).
3.3 Porosity
Porosity stems from entrapped bubbles (due to turbulent flow of the melt), or from
dissolved gas segregating and coming out of solution during solidification (the bubbles
effectively being “precipitates” of gas). Porosity is more severe if liquid metal cannot feed
into the last regions to solidify, leading to large cavities. Interconnected 'sponge porosity'
can cause castings to leak in service.
Dissolved gas can be partially extracted by holding the melt under vacuum (expensive).
Alternatively, it can be converted into solid particles (e.g. oxides) during solidification,
e.g. ‘killing’ a steel means adding Al powder (or other oxide former), to produce Al2O3
particles, effectively removing oxygen from solution.
Large scale: mainly relevant to bulk cast ingots, subsequently rolled to stock sizes.
In some cases, a controlled amount of porosity can be beneficial:
(a) Degassed steel
(vacuum-treated or
killed):
(b) Balanced “semikilled” steel:
Distributed bubbles
within the interior,
balancing the volumetric
contraction. Whole ingot
can be rolled, and porosity
is flattened out during
subsequent hot rolling.
Ingot contracts without
further metal feeding,
leading to macroporosity
and an internal “pipe”.
Ingot must be cropped,
leading to high scrap
fraction.
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Over 90% of steel in Britain is produced by
continuous
continuous casting,
casting, in
in which
which macroporosity
macroporosity
problems
are
eliminated.
The
problems are eliminated. The steel
steel is
is vacuum
vacuum
degassed before casting.
degassed before casting.
Ingot structure:
- part columnar, part equiaxed.
- no macroporosity (liquid “sump”
connected to incoming liquid metal).
Micro scale: Porosity on the scale of the dendrite arms is a real “fingerprint” of cast
structures, and can be important in limiting mechanical properties.
Example cast microstructures: e.g. Al-4% Cu
As-cast:
Homogenised:
Grain size
unchanged – but
grains don’t look
like dendrites any
more: impurities
have redistributed.
Dark spots within
the grains are the
porosity trapped
between the
dendrite arms.
Dark regions are
inter-dendritic
regions and grain
boundaries.
Dark colour arises
from etching of
impurities, and
from porosity.
Each dendrite makes
one grain.
Note that the porosity remains after homogenization: the pores are filled with gas, and for
the pores to close the gas has to diffuse away through the solid. Most of the gas is in the
form of molecules rather than single atoms, and the diffusion rate for these is very small
(since molecules are much bigger).
Porosity can be removed by deformation e.g. rolling: OK for an ingot; not for a near-net
shape casting! Hot isostatic pressing (HIPing) can be used to remove porosity (see
Powder Processing lectures) – costly, but provides marked improvement in mechanical
properties. Easiest for Al and Mg alloys, where HIPing temperature required is
comparatively low (so relatively economical).
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3.4 Casting Alloys
For near-net shape castings, material considerations include:
Melting temperature, solidification temperature range, shrinkage.
Common casting alloys are often close to eutectic composition.
Advantages:
Low melting temperature:
Cheaper (lower heating cost, faster production rate)
Low solidification (“freezing”) range: (i.e. from liquidus  solidus temperatures)
High fluidity
Easier to feed: lower shrinkage, lower porosity
The “freezing range” is important for casting integrity: liquid feeding will be slow in the
semi-solid “mushy zone”, and the mixture of liquid and solid has very little strength so
tears easily due to contraction stresses.
Two-phase material so scope for improved strength (fine-scale hard second phase)
Example cast microstructures: Al-13%Si eutectic (basis of many Al casting alloys)
(Phase Diagram: see Materials Databook)
The silicon forms a network structure of hard and brittle needles in the soft Al matrix.
Considerably improved strength, toughness and ductility are achieved by poisoning the
growth mechanism of the brittle non-metallic phase: adding 0.01% sodium immediately
before casting to modify the growth to give finer, more rounded particles of silicon.
Na modified (Al removed by etch):
much finer, rounded Si phase
Unmodified Al-Si eutectic
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Grey Cast Iron
Cast irons still have high-volume markets.
Example near-eutectic alloys: Fe + 3-4%C (eutectic at 1130ºC).
Microstructure: ferrite + graphite (+ iron carbide, depending on composition/heat treatment).
Graphite ‘morphology’: interconnected network of plates
(a) Schematic of graphite
(b) Scanning Electron Micrograph
of deep-etched sample (Fe removed)
The graphite plates are weak, and act like
cracks – so unmodified cast iron has poor
tensile properties. The flake size is
dependent on the cooling rate: faster
cooling gives more nucleation, smaller
flakes and higher strength.
The graphite flakes are good for damping
mechanical vibration (so useful for
machine tool mountings), and cast iron
can be machined without lubricant (as the
graphite provides internal lubrication).
(c) Optical micrograph (polished surface)
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Improved mechanical properties can again
be achieved by poisoning: changing the
growth mechanism of the (non-metallic)
graphite. In this case, the addition of a
small amount (approx. 0.5wt%) of
magnesium or cerium causes the graphite
to form as nodules, significantly
improving the strength and toughness.
As-cast structure of modified cast iron:
‘Nodular’ or ‘Spheroidal Graphite (SG)’
cast iron.
Dimensional changes on solidification of cast iron
In carbon steels and “white” cast irons, the carbon forms cementite Fe3C. However, in
grey cast irons, much of the carbon forms graphite (as above). Graphite is a low density
phase, so this counteracts the contraction of the iron on solidification. For a specific
composition of grey cast iron (just hypo-eutectic) there is a zero volume change on
solidification – useful for producing castings directly to final size and shape.
% volume
change on
solidification
Fe3C 25